1. Field of the Invention
The present invention relates to multilayer capacitors, and in particular, to a multilayer capacitor that is a multi-terminal type that reduces an equivalent series inductance (ESL).
2. Description of the Related Art
When impedances of a power line and the ground in a power supply circuit cause large fluctuations in the power line, the operation of a driven circuit becomes unstable, interference between circuits via the power supply circuit occurs, and oscillation occurs. Accordingly, a decoupling capacitor is normally connected between the power line and the ground. The decoupling capacitor has a role of suppressing fluctuation in power-supply voltage and interference between circuits by reducing the impedance between the power line and the ground.
In recent years, in communication devices such as cellular phones and information processing devices such as personal computers, signal speed has been increased in order to process large amounts of information, and clock frequencies of ICs (integrated circuits) in use have also been increased. Accordingly, noise that includes many harmonic components is easily generated, and, in an IC power supply circuit, stronger decoupling needs to be performed.
To enhance a decoupling effect, use of a capacitor having good impedance frequency characteristics is effective. Capacitors adapted for the purpose include a multilayer capacitor. The multilayer capacitor has a good absorption effect in a broad frequency range compared with an electrolytic capacitor since the multilayer capacitor has a small ESL.
In general, a multilayer capacitor used as a decoupling capacitor for an IC is disposed in the vicinity of the IC. This is because, when voltage fluctuation occurs in a power supply line, by rapidly supplying electric charge from the capacitor to the IC, the start of the IC is prevented from being delayed.
However, when the capacitor charges or discharges, a counter electromotive force, represented by dV=L·di/dt, is generated in the capacitor. When counter electromotive force dV is large, supply of electric charge to the IC is delayed. In the case of changing the IC clock frequency to a high frequency, a current fluctuation amount per unit, represented by di/dt, tends to increase. In other words, to reduce counter electromotive force dV, inductance, represented by L, needs to be decreased. Inductance L includes an inductance of a pattern from a power supply pin of the IC to the capacitor, an inductance of a pattern from the capacitor to the ground, and an ESL in the capacitor. In circuit design, there is a limitation in reducing the inductances of the patterns. Accordingly, it is requested that the ESL of the capacitor be more reduced.
Accordingly, as multilayer capacitors in which the ESL is more reduced, those disclosed in Japanese Unexamined Patent Application Publication Nos. 2002-151349 (see particularly FIGS. 3, 6, and 8), 2004-103883 (see particularly FIG. 1), and 2006-32904 (FIGS. 4, 7, and 13) have been proposed.
FIGS. 13A to 16B are plan views showing an internal structure of a multilayer capacitor by using sections including electrodes. FIGS. 13A and 13B show such a structure disclosed in Japanese Unexamined Patent Application Publication No. 2002-151349. FIGS. 14A to 14D show such a structure disclosed in Japanese Unexamined Patent Application Publication No. 2004-103883. FIGS. 15A to 16B show such a structure disclosed in Japanese Unexamined Patent Application Publication No. 2006-32904.
FIGS. 13A to 16B show, in common, a substantially rectangular parallelepiped laminate 6 which is formed by a plurality of laminated dielectric layers 1 and which includes first and second side surfaces 2 and 3 facing each other, and third and fourth side surfaces 4 and 5 facing each other. In FIGS. 13A to 16B, examples of current flows are indicated by dotted line arrows.
First, referring to FIGS. 13A and 13B, Japanese Unexamined Patent Application Publication No. 2002-151349 discloses a multi-terminal multilayer capacitor 8 in which a total of eight external terminal electrodes 7 are formed on the first and second side surfaces 2 and 3 of the laminate 6 which face each other. In the multilayer capacitor 8, an internal electrode 9 that leads to the first side surface 2 is disposed so as to face, in a coplanar manner, an internal electrode 10 that leads to the second side surface 3 facing the first side surface 2. Similarly, an internal electrode 11 is disposed so as to face an internal electrode 12 in a coplanar manner. According to this structure, currents that flow in the internal electrodes 9 to 12 are opposite in facing portions, whereby magnetic fields generated around the currents cancel one another, so that an ESL of the multilayer capacitor 8 decreases.
Next, referring to FIGS. 14A to 14D, Japanese Unexamined Patent Application Publication No. 2004-103883 discloses a multi-terminal multilayer capacitor 14 in which a total of eight external terminal electrodes 13 are formed on first to fourth side surfaces 2 to 5. In the multilayer capacitor 14, internal electrodes 15, 16, and 17 which face one another in a coplanar manner, and internal electrodes 18, 19, and 20 which face one another in a coplanar manner alternately lead to first and second side surfaces 2 and 3 of a laminate 6 which face each other. In addition, the internal electrodes 15 to 17, and 18 to 20 face common internal electrodes 21 and 22 in the laminating direction to form capacitors. The common internal electrodes 21 and 22 respectively lead to third and fourth side surfaces 4 and 5 of the laminate 6 which face each other. According to this structure, currents that flow in the internal electrodes 15 to 20 are opposite in facing portions, whereby magnetic fields generated around the currents cancel one another, so that an ESL of the multilayer capacitor 14 decreases.
Next, Japanese Unexamined Patent Application Publication No. 2006-32904 discloses various sorts of multi-terminal multilayer capacitors. For example, as shown in FIGS. 15A and 15B, a multi-terminal multilayer capacitor 24 in which a total of eight external terminal electrodes 23 are formed on first and second side surfaces of a laminate 6 which face each other. In the multilayer capacitor 24, in U-shaped internal electrodes 25 and 26 that face each other in a coplanar manner, and U-shaped internal electrodes 27 and 28 that face each other in a coplanar manner, currents are opposite in facing portions, whereby magnetic fields generated around the currents cancel one another, so that an ESL of the multilayer capacitor 24 decreases.
Japanese Unexamined Patent Application Publication No. 2006-32904 also discloses the multi-terminal multilayer capacitor 24a shown in FIGS. 16A and 16B. In the multilayer capacitor 24a, internal electrodes 25a and 26a face internal electrodes 27a and 28a in the laminating direction, and currents in facing portions are orthogonal, whereby magnetic fields generated around the currents are weakened, so that an ESL of the multilayer capacitor 24a decreases.
The structures described in Japanese Unexamined Patent Application Publication Nos. 2002-151349, 2004-103883, and2006-32904 have the following problems to be solved.
In the structure described in Japanese Unexamined Patent Application Publication No. 2002-151349, the length of facing edges of the internal electrodes 9 and 10, or the length of facing edges of the internal electrodes 11 and 12 is subject to the distance between lead portions of each of the internal electrodes 9 and 12, that is, a pitch between the external terminal electrodes 7. The pitch between the external terminal electrodes 7 is set to be a predetermined distance or greater when considering a problem of short-circuiting, while the pitch between the external terminal electrodes 7 needs to be shortened to a predetermined distance or less in view of downsizing. Accordingly, in actual design, there is a limitation in pitch between the external terminal electrodes 7. As a result, there is a limitation in lengths of the facing edges of the internal electrodes 9 and 10 and the facing edges of the internal electrodes 11 and 12. Therefore, a method of adjusting the length of a current path by adjusting the lengths of the facing edges cannot be simply employed, and adjustment of an equivalent series resistance (ESR) of the multilayer capacitor 8 is not always facilitated.
The ESR affects stabilization of a power supply circuit. In other words, in a multilayer capacitor having a reduced ESL, an increased number of terminals increases the number of lead portions of internal electrodes. As a result, the ESR tends to be low. When the ESR is too low, there are problems in that inductance of a peripheral circuit causes a resonance phenomenon, so that power supply voltage greatly drops, or so that damped oscillation, such as ringing, easily occurs. Accordingly, it is necessary to increase the ESR to some extent. However, in the structure described in Japanese Unexamined Patent Application Publication No. 2002-151349, as described above, ESR adjustment is not always facilitated.
In addition, in the structure described in Japanese Unexamined Patent Application Publication No. 2002-151349, two separate capacitor portions are formed. When increasing the number of capacitor portions, a new capacitor portion cannot be added in a width direction of the laminate 6, that is, a direction in which the third and fourth side surfaces 4 and 5 extend. Therefore, in a longitudinal direction of the laminate 6, that is, a direction in which the first and second side surfaces 2 and 3 extend, a new capacitor portion is added. In this case, similarly to the shown internal electrodes 9 to 12, one or more pairs of internal electrodes are formed, so that the number of capacitor portions increases by a multiple of 2, and the number of external terminal electrodes 7 increases in the longitudinal direction by a multiple of 4. Accordingly, design flexibility is low, and, in addition, there is a problem in that, when adding capacitor portions, the laminate 6 is too elongated to cause a flexural strength of the laminate 6 to be low.
Next, in the structure described in Japanese Unexamined Patent Application Publication No. 2004-103883, the internal electrodes 21 and 22 that face, in common, the internal electrodes 15 to 17 or 18 to 20 include a portion that is not related to capacitor formation. From a capacitor-formation viewpoint, it may be said that an electrode material is not effectively used. In addition, since a bonding area of adjacent dielectric layers 1 for positioning each of the common internal electrodes 21 and 22 at an interface is small, there is also a problem in that a structural defect, such as delamination, easily occurs in the laminate 6. Further, the common internal electrodes 21 and 22 lead to the third and fourth side surfaces 4 and 5, which differ from the first and second side surfaces 2 and 3 from which the internal electrodes 15 to 20 lead. Thus, it is necessary to form the external terminal electrodes 13 on each of four side surfaces 2 to 5, so that there is a problem in that design flexibility is low.
Next, the multilayer capacitor 24, shown in FIGS. 15A and 15B, having the structure described in Japanese Unexamined Patent Application Publication No. 2006-32904, has problems similar to those in the structure described in Japanese Unexamined Patent Application Publication No. 2002-151349. In addition, similarly to the case of Japanese Unexamined Patent Application Publication No. 2004-103883, in the multilayer capacitor 24a shown in FIGS. 16A and 16B, the internal electrodes 25a to 28a include portions that are not related to capacitor formation. Accordingly, from a capacitor formation viewpoint, there is a problem in that an electrode material is not effectively used. In addition, similarly to the case of the Japanese Unexamined Patent Application Publication No. 2004-103883, in the structure described in Japanese Unexamined Patent Application Publication No. 2006-32904, a bonding area of adjacent dielectric layers 1 for positioning each of the internal electrodes 25 to 28 or 25a to 28a at an interface is small, so that there is a problem in that a structural defect, such as delamination, easily occurs in the laminate 6.